Fix: syncVnodeTree should nullify vnode for removed children

Input from Gemini 3 Pro:

✦ I attempted to implement the fix for this issue but encountered regressions that caused VDOM update cycles to time out in tests.

Learnings & Analysis:

1. Logical Child Iteration: Switching from getChildren(vnode) to ComponentManager.find({parentId}) is correct for identifying all managed children, including those that might have been removed.
2. Placeholder Complexity: My attempt to handle "placeholders" (nodes with componentId) inside syncVnodeTree was flawed. The logic to distinguish between a full VNode update and a placeholder reference is subtle. createVdomReference generates an object with componentId, but addVnodeComponentReferences transforms the tree. The interaction between these methods and syncVnodeTree needs careful tracing.
3. Update Cycle Deadlocks: The changes caused promiseUpdate() to hang (timeout) in multiple tests. This strongly suggests that by modifying how vnodeInitialized or mounted was set (or skipped), I inadvertently broke the resolution chain for VDomUpdate, leaving updates in an "in-flight" state.
4. The "Silent" Update: The _vnode update in syncVnodeTree is critical. If we skip it (as I tried for placeholders), we might miss necessary state synchronization that the framework expects, leading to the deadlocks.

Recommendation for Next Attempt:

• Focus strictly on the missing children case first.
• Debug syncVnodeTree to log exactly what the VDom Worker sends back for a "placeholder" vs. a "full" node.
• Ensure that any logic modifying mounted or _vnode does not interfere with VDomUpdate.executeCallbacks.

Input from Gemini 3 Pro:

✦ I have implemented the fix for this issue.

Changes:

1. Modified src/mixin/VdomLifecycle.mjs to use a Hybrid Two-Pass Strategy:

   • Pass 1 (Update): Iterates over visibleChildren (found in the new VNode structure). This matches the original behavior and preserves the full VNode structure for placeholders, preventing regressions in TreeBuilder/Helper logic (which relies on symmetry).
   • Pass 2 (Unmount): Iterates over ALL logical children (ComponentManager.find). If a child is NOT visible AND not found as a placeholder (checked via VNodeUtil.find), it is correctly unmounted (_vnode = null, mounted = false).

2. Added a regression test test/playwright/unit/vdom/HiddenChildren.spec.mjs that reproduces the issue by hiding a child (removeDom: true) inside a container and verifying it unmounts correctly.

Verification:

• HiddenChildren.spec.mjs passes (Child is unmounted).
• RealWorldUpdates.spec.mjs passes (No extra deltas/regressions).
• Full npm run test-unit passes (250 tests).

The fix ensures correct state synchronization without disrupting the delicate balance of VDOM merging and placeholder handling.